Astrophysics > High Energy Astrophysical Phenomena

Abstract: It is commonly believed that the optical/UV and X-ray emissions in luminous
AGN are produced in an accretion disk and an embedded hot corona respectively.
The inverse Compton scattering of disk photons by hot electrons in the corona
can effectively cool the coronal gas if the mass supply is predominantly via a
cool disk like flow as in BHXRBs. Thus, the application of such a model to AGNs
fails to produce their observed X-ray emission. As a consequence, a fraction of
disk accretion energy is usually assumed to be transferred to the corona. To
avoid this assumption, we propose that gas in a vertically extended
distribution is supplied to a supermassive black hole by the gravitational
capture of interstellar medium or stellar wind material. In this picture, the
gas partially condenses to an underlying cool disk as it flows toward the black
hole, releasing accretion energy as X-ray emission and supplying mass for the
disk accretion. Detailed numerical calculations reveal that the X-ray
luminosity can reach a few tens of percent of the bolometric luminosity. The
value of $\alpha_{\rm ox}$ varies from 0.9 to 1.2 for the mass supply rate
ranging from 0.03 to 0.1 times the Eddington value. The corresponding photon
index in the 2-10 keV energy band varies from 1.9 to 2.3. Such a picture
provides a natural extension of the model for low luminosity AGN where
condensation is absent at low mass accretion rates and no optically thick disk
exists in the inner region.